How to save shadow catcher image as png?
You can use cycles to bake the shadow into a texture file. To do so, create a shadow catcher, you can do this by clicking the object properties button after selecting your plane. (It's the orange square) Then once it's a shadow catcher, and you have set up the lights to get the shadow you want, go to render settings, open bake, and select shadow. Now open the material you are using for your shadow catcher and add an image texture node. Press the create new button, choose your resolution and name. Then select it in the node editor. Now just click bake.